Alloying metal cations in perovskite nanocrystals is a new route to controlling hot carrier cooling

Hot carrier cooling is slowed down upon alloying tin in lead-halide perovskite nanocrystals through the engineering of carrier-phonon and carrier-defect interactions.

sparking broad discussions about the prospect of hot carrier perovskite solar cells.
The overall HCC mechanism can be divided into a series of steps in any perovskite system, including the one studied by Dai et al. 18 (Fig. 1).First, the absorbed photons generate free electrons and holes with a non-equilibrium distribution of excess energies.These charge carriers undergo thermalisation through rapid (10-100 fs) carriercarrier scattering events and establish an equilibrium distribution characterised by an effective hot carrier temperature.These thermalised hot carriers exchange energy with the lattice on a 0.1-1ps timescale via carrierphonon interactions and eventually cool down to the lattice temperature.Owing to the polar nature of perovskite semiconductors, the dominant pathway of relaxation is considered to be the Fröhlich interaction between hot carriers and longitudinal optical (LO) phonons, which ceases once the excess energy of the hot carriers falls below that of the LO phonon energy gap.Thereafter, the emitted LO phonons decay into daughter longitudinal acoustic phonon branches (through Klemens pathway), which spread the heat across the material (or device) on the macroscale, depending on the thermal conductivity of the system.As evident from the mechanism indicated above, a viable route towards slowing down HCC could be in carefully blocking one or more of the intermediate energy dissipation steps.
As the first stage of HCC is mainly governed by their coupling to LO phonons derived from the Pb-X sub-lattice, one would expect similar HCC dynamics across the lead-halide perovskite family.However, differences in HCC have been observed for hybrid (A = methylammonium, formamidinium) perovskites compared to their allinorganic Cs-based counterparts and polaron formation, thought to depend on the nature of the A-cation, has been suggested as a possible explanation for this 6 .In perovskite systems, HCC can also be slowed by the hot phonon bottleneck (HPB) effect that becomes prominent under high carrier densities.Conceptually, this can be regarded as the increased competition for a finite availability of cold LO phonons into which the carriers may deposit their excess energy [4][5][6][7][8][9][10][11][12][13] .Furthermore, acoustic-optical phonon upconversion could also contribute to the slowing down of HCC, as reported by Yang et al. 8 .
HCC in perovskite materials, particularly the quantumconfined nanoscale systems, may also be affected by Auger recombination (at high carrier density, usually ~10 19 cm −3 ), wherein the energy released under the recombination of one electron-hole pair is transferred to a nearby third carrier.This leads to the generation of further hot carriers (known as 'Auger heating'), opening up another route to sustaining their population over longer timescales 4,19 .
Apart from the major influence of phonons and carriercarrier interactions, the role of intra-bandgap electronic states such as surface traps or interstitial defects on HCC dynamics remains debated and underexplored 11,18,21,22 .For any given system, small variations in the reported hot carrier lifetime are perhaps due to the lack of proper estimation of the traps present in the system.This intuitively points towards the identification of each mechanistic route for HCC, and ways to control them require intense research in perovskite composition space.While there is great debate over whether partial or complete replacement of Pb by the non-toxic Sn could offer similar optoelectronic properties, including HCC, rapid oxidation of Sn 2+ to Sn 4+ introduces another challenge to the stability of this material.In prior work, Dai & colleagues employed stable pure Sn-based NCs as a testbed to study hot carrier dynamics 17 , and now show that Sn-Pb alloying provides an additional dimension to control HCC 18 .
In this work 18 , Dai et al. introduced Sn in MA and Csbased lead iodide perovskite nanocrystals and followed the conventional procedure of extracting HCC dynamics from transient absorption spectra 4,5,7,9 .Based on the timedependent spectral narrowing of the band-edge bleach they tracked the dynamics of hot-carrier temperature within the first 100 ps after photoexcitation.The decay of hot carrier temperature from 1000s to 100s K could be described by a single exponential function at carrier densities lower than ~10 18 cm −3 , becoming biexponential at >10 18 cm −3 .The dominant sub-ps decay (τ 1 ) was assigned to carrier-LO phonon coupling, while the secondary few-ps time component (τ 2 ) was related to the HPB effect.The researchers observed that τ dielectric constant, leading to phonon screening, which in turn slowed the τ 1 decay.Based on previous reports, Dai et al. proposed that the collective contributions of suppressed Klemens decay (due to a greater optical-acoustic phonon energy gap), as well as lower thermal conductivity, slow down τ 2 with increasing Sn content 18 .Dai et al. further revealed that shorter τ 1 and τ 2 for all the Pb-Sn alloy and pure Sn-based NCs compared to the pure Pb-based NCs are due to the influence of the competing hot carrier trapping process in the Sn-based systems.Indeed, these former systems possess lower photoluminescence quantum yields and shorter band-edge carrier lifetimes due to the abundance of trapping sites.Interestingly, fully inorganic (CsSn x Pb 1-x Br 3 ) NCs exhibited an even shorter hot carrier lifetime compared to their hybrid counterpart, possibly due to the presence of more trapping centres in the all-inorganic systems.This is further corroborated by the slowing down of τ 1 and τ 2 , and higher hot carrier temperature when trap states were passivated via Na-doping 18 .
The study by Dai et al. 18 opens space for reflection and consolidation of recent developments in the field of HCC in perovskite materials.As far as the HCC mechanism is concerned, it still remains unclear why the nanoscale counterparts of Pb-Sn alloyed systems demonstrate reduced HPB compared to bulk analogues 11 .The answer may lie in the nature of the traps, and it would be of immediate interest to identify and characterise the origin of electronic defects contributing to the acceleration of HCC dynamics.Importantly, it would be also of interest to clarify whether hot carrier trapping can significantly compete with the slowing route of HPB.
Due care also needs to be taken rationalising and disentangling the role of the HPB from Auger heating effects.Two-pulse ('pump-probe') spectroscopic techniques do not provide independent control of the hot and cold carrier sub-populations, and in this regard, complementary three-pulse ('pump-push-probe') spectroscopic approaches might appear very helpful 11,16,23 .This knowledge would further facilitate the development of design principles towards slowing HCC at excitation intensities comparable to solar illumination.
Finally, while a respectable decrease in HCC rate is achieved, reports on directly harvesting these short-lived hot carriers remain limited 23,24 .We hope that further studies will open up the path towards applying the concepts discussed above in the practical development of hot carrier solar cells.

Fig. 1
Fig. 1 Hot carrier cooling mechanisms in perovskites, including Pb-Sn alloy NCs.At low-to-moderate carrier density, HCC is limited by trapping and the hot phonon bottleneck effect; while Auger heating dominates at high carrier density (DOS = density of states)